This application is based on and claims the priority under 35 U.S.C. xc2xa7119 of German Patent Application 199 48 229.2, filed on Oct. 7, 1999, the entire disclosure of which is incorporated herein by reference.
The invention relates to a high-frequency ion source including a discharge chamber into which a gas is supplied, and a high-frequency coil surrounding the chamber for ionizing the gas.
The invention relates to a high-frequency ion source and particularly an ionic propulsion engine for a spacecraft, including a discharge chamber into which a gas is supplied, and a high-frequency coil surrounding the chamber for ionizing the gas.
High-frequency ion sources are used in space technology as engines in space vehicles. The assignee of the present application has developed a high-frequency ion engine comprising a discharge chamber (also called a discharge container herein) at one end, with a gas inlet for supplying into the discharge container a gas to be ionized in the discharge container, and a source for the gas to be ionized, said source being connected to the gas inlet. The engine further comprises a high-frequency coil surrounding the discharge container, a high-frequency generator connected to the high-frequency coil, for generating a high-frequency electromagnetic alternating field which ionizes the gas present in the discharge container, and an acceleration grid arranged at the open end of the discharge container and connected to an acceleration voltage source. In this high-frequency ion engine, which is known as an RIT (radio-frequency ion thruster), in the discharge container, which is made of an electrically non-conductive material, a high-frequency field is generated by means of the high-frequency coil surrounding the discharge container. This high-frequency field ionizes a propellant present in the discharge container, preferably an inert gas such as xenon. To ignite the discharge, free electrons supplied by an external electron source are accelerated through the high-frequency field and collide with neutral propellant particles, i.e. inert gas atoms. At every collision an electron is ejected from the neutral atom, and the atom is positively ionized. The electrons which are released are again accelerated and collide with other neutral atoms, thus starting a process of ionization, and generating a plasma comprising ions, electrons and neutral propellant. The fraction of ions in the plasma is determined by the output provided by the high-frequency field. When the discharge has ignited, it sustains itself; there is no need for any external supply of electrons. Thrust is generated by means of a voltage applied to the acceleration grid by the acceleration voltage source. Ions present near the acceleration grid are accelerated by the electrical field generated by means of the acceleration voltage, with a focused ion beam being formed. Generally, a neutralizer is used which supplies electrons to the ion beam during thrust operation, so as to prevent negative charging of the engine.
Previously, in such a high-frequency ion engine, a discharge chamber or container of cylindrical shape was used. In such a container the gas inlet for the gas to be ionized is located in a plane or slightly conical end surface (called the xe2x80x9cclosed endxe2x80x9d surface herein) of the cylinder, which closed end surface is opposite the open end of the discharge container. It should be understood that the term xe2x80x9cclosed endxe2x80x9d is simply a convenient shorthand name for the end of the container opposite the xe2x80x9copen endxe2x80x9d, and it does not require this end to be completely xe2x80x9cclosedxe2x80x9d. To the contrary, for example, the gas inlet opening may be provided in the xe2x80x9cclosed endxe2x80x9d. The above-mentioned acceleration grid for accelerating the ion beam is provided at the opposite open end surface. The acceleration grid comprises two to three thin plates made of an electrically conductive material, with a plurality of holes provided therein, with said holes being arranged so as to form extraction channels which focus and accelerate the ions. The plates forming the acceleration grid can be plane or slightly curved in the extraction region. In the known configuration, the high-frequency coil surrounds the cylindrical part of the discharge container.
A high-frequency ion engine as described above is for example known from published European Patent Application EP 0,560,742. Analogous arrangements are described in published European Patent Application EP 0,537,123, German Patent Laying-Out Document DE 26 33 778 and Japanese Patent Laying-Open Document JP 2-230971. By contrast, U.S. Pat. No. 5,170,623 discloses a hybrid drive system formed by a combination of a combustion engine and an electromagnetic engine. The combustion gases of a usual rocket combustion engine are additionally accelerated by means of an electromagnetic coil in the region of the expansion nozzle of the rocket engine.
It is an object of the invention to create an improved high-frequency ion source such as an ion engine, and a method of operating the same, with an increased structural strength, reduced weight, improved ionization, increased efficiency and increased thrust to mass ratio. The invention further aims to avoid or overcome the disadvantages of the prior art, and to achieve additional advantages, as apparent from the present specification.
The above objects have been achieved according to the invention in a high-frequency ion source comprising a discharge chamber or container open at one end, with a gas inlet for discharging into the discharge container a gas to be ionized in the discharge container, and a source for the gas to be ionized, said source being connected to the gas inlet. The ion source further comprises a high-frequency coil surrounding the discharge container, a high-frequency generator connected to the high-frequency coil, for generating a high-frequency electromagnetic alternating field which ionizes the gas present in the discharge container, and an acceleration grid arranged at the open end of the discharge container and connected to an acceleration voltage source. The invention provides for the discharge container to have a tapered shape in longitudinal section, which shape tapers toward the closed end opposite the open end. Further according to the invention, the high-frequency coils of the discharge container at least partly surround the discharge container in the tapered section.
The inventive configuration of the high-frequency ion source provides a number of significant advantages. One advantage is the increased mechanical strength of the discharge container at lower weight. There is a further advantage in that an increased field strength is attained in the region of the gas inlet, due to the small diameter of the coil in this region, leading to improved ionization of the propellant and improved mass efficiency. A further advantage is provided by a more even distribution of the plasma density across the container radius in the region of the acceleration grid and thus increased extractable ion streams and improved thrust. A further advantage is provided by a reduction in wall losses, i.e. of ions which neutralize on the wall without being accelerated, as a result of a reduced wall surface in relation to the volume of the discharge container. It is also advantageous that with the same surface of the interior wall and the same diameter in the region of the acceleration grid, the discharge container can be greater in length, so that the path length between the gas inlet and the acceleration grid is longer and thus the probability of ionization of the propellant atoms is improved.
Finally, in the case of a particular embodiment according to the invention, in contrast to a cylindrical shape of the discharge container, with the same length of the discharge container, the high-frequency coil will experience lesser rheostatic losses because the average diameter is smaller and thus the coil wire is shorter.
An advantageous embodiment of the invention provides for the discharge container to be of frusto-conical shape in longitudinal section.
Another advantageous embodiment of the invention provides for the discharge container to be of frusto-conical cylindrical shape in longitudinal section, with a cylindrical part facing the open end, and a frusto-conical part facing the closed end opposite the open end.
Another advantageous embodiment of the invention provides for the discharge container to be conical frustum-shaped in longitudinal section.
A further advantageous embodiment of the invention provides for the discharge container to be nozzle-shaped in longitudinal section, whereby the nozzle-shape tapers with an increasing curvature.
An advantageous embodiment of the invention provides for the discharge container comprising a plane, slightly curved and conical end surface at the closed end opposite the open end.
Preferably, the gas inlet in the closed end surface opposite the open end, leads into the discharge container.
An advantageous embodiment of the invention provides for the high-frequency coil to be configured as a single-layer coil.
Preferably the discharge container comprises an electrically non-conductive solid material of little high-frequency loss in the range between 0.5 MHz and 100 MHz, in particular made of quartz, aluminum oxide, other ceramic material, Vespel, boron nitride or Macor.
Preferably the discharge container is surrounded by a housing comprising a conductive material, in particular metal. Preferably the shape of the housing matches that of the discharge container. Advantageously, the housing comprises a cylindrical or conical/cylindrical shape. An advantageous embodiment provides for the housing to surround the discharge container at a distance of 1 to 4 cm.
Advantageously, the high-frequency coil is excited by a resonance frequency of 0.5 MHz to 5 MHz. Preferably the high-frequency generator comprises a phase-locked loop (PLL) control circuit.
Furthermore, the invention provides a method for operating a high-frequency ion source of the type described above, in which the high-frequency coil is operated in resonance, both before ignition of the discharge in the discharge container and in idling operation after ignition of the charge, but without applying an acceleration voltage to the acceleration grid, as well as during thrust operation with acceleration of the ions through the acceleration grid.